This invention relates to an easily transportable unit comprising a container having a base adapted to be picked up and loaded onto or into transport means for transporting the unit, as well as adapted to be unloaded from the transport means, without a requirement of due care. The container is packed with equipment which provides pure water as "permeate" from reverse osmosis ("RO") modules which separate potable water from contaminated water unfit for human use. The specific requirement of the unit is that the RO modules deliver permeate in a relatively small volume/unit time, irrespective of how isolated the environment in which only contaminated fresh water, brackish water, or sea water is to be found.
As will presently be evident, the maximum dimensions and weight of the unit is specified, as is the wide range of conditions under which it must operate successfully. The problem is to use major components which are readily available and to pack them into the specified dimensions without sacrificing reliability of the unit.
RO modules in various systems are routinely used to produce high purity water by pumping source water at a pressure above the osmotic pressure of the dissolved substances in the water, through a semipermeable membrane. Permeate passes through the membrane but the contaminants remain in a "concentrate" stream which is discharged. Even to produce only 20 gpm of permeate from contaminated water, requires feeding the contaminated water at such high pressure that the mass and weight of the modules housing RO elements with adequate surface area and associated equipment is unexpectedly high. If the ROWPU is to be self-contained it must also carry its own tanks for back-flushing (or "back-washing"--the terms are used synonymously) filters and for chemical cleaning solution to clean the RO elements, along with a generator for generating electricity, and related equipment with associated instrumentation.
In addition to the RO modules and tanks, the equipment packed in the container, referred to as a "purification container" or "PC" for brevity, includes pumps and a combination of coarse and fine prefiltration means, on-board pressurized back-flushing means for a coarse screen filter, and appropriate valving and instrumentation. The modules are connected so that they may be operated in either a single pass or double pass mode, depending upon the quality of the water source, and the amount of the permeate desired.
More specifically, the RO equipment is specifically directed to fulfill the need of a military personnel unit which requires from about 10 gpm (gallons/minute) but no more than 20 gpm, typically about 15 gpm, of permeate produced in a totally self-contained PC which is demountably disposed on a truck for transportation, or even carried in an airplane or by a helicopter and air-delivered, so that the PC delivers potable water from whatever source of water which may be available at any location where the PC is delivered. Under conditions of conflict, or in the event of a major accident at a nuclear facility, when water may be contaminated with nuclear, biological and chemical ("NBC" for brevity) warfare agents the equipment is able to deliver potable water at reasonable cost.
Since in an isolated environment there is no available electrical power, it will be evident that the PC must provide its own, therefore includes a hydrocarbon fuel-powered (diesel fuel) generator. What is less evident is that the PC must be fully assembled and easily operable under difficult conditions, since it is unlikely that skilled personnel will be available to assemble and operate the equipment. In other words, the overriding consideration is that transportability of the PC must be as a single unit with all components preassembled and ready to operate with only rudimentary instructions to an operator.
The armed services were supplied with a reverse osmosis water purification unit described in an article titled "Reverse Osmosis Development of a High Technology Water Treatment Capability for the Canadian Forces" by Capt M. J. Hauschild in UBIQUE No. 35, pg 32-37, Sept '90. This unit was referred to as an ADROWPU (acronym for Air Deliverable Reverse Osmosis Water Purification Unit), because it was hoped it could be delivered by helicopter. The term "purification container" or PC is used herein substantially synonymously with the term ADROWPU. The components of the system schematically represented in the article, used two banks of RO modules arranged to operate in a double pass mode.
By "double pass mode" is meant that water to be filtered is flowed through two RO modules, in practice, two banks of modules, in two (first and second) passes. Permeate from the first pass (referred to as "first permeate") is flowed as feed to the second bank to be filtered in a second pass, so as to yield a second permeate different in quality from the first permeate. Thus, if the rejection in the first pass is 80% so that only 20% of the incoming water (to the first bank) is withdrawn as first permeate, this first permeate is then fed to the second bank and again filtered in the second pass. If the rejection in the second pass is the same as that in the first pass, namely 80%, then only 4% of the incoming water (to the first bank) is withdrawn as second permeate from the second pass.
Details of the ADROWPU referred to in the article are substantially as shown in FIG. 1 of the appended drawing. The raw water pump supplied water to a process pump. To remove suspended solids a combination of screen filters (20 mesh and smaller mesh), and cartridge microfilters (100 .mu.m and 50 .mu.m) were used. Other conventional removal of suspended solids, such as by adding coagulant, then flowing through multi-media filters and cartridge microfilters are also effective. This ensured that suspended solids are removed. A solids-free feedwater was fed to the banks of RO modules.
The last step during normal operation was to add calcium hypochlorite to disinfect the RO permeate so as to maintain a chlorine residual of 5 mg/L. If required, an NBC post treatment with granular activated carbon for chemical warfare agents, and, an ion exchange column for nuclear warfare agents is added to the overall package.
Though the aforementioned ADROWPU delivered the desired permeate at the desired flow rate, it was deemed inadequate for service because of at least two problems. First, operation was interrupted too frequently because the unit was shut down to replace the cartridge filter.
Other transportable RO module systems have been used by the armed forces. A particular example is described in an article titled "The Army Water Supply Program" by Thomas . Bagwell, Jr., U.S. Army Belvoir Research and Development and Engineering Center, September 1990. This unit was a 600 gal/hr reverse osmosis water purification unit (referred to as a "ROWPU" in the article) in which the components were arranged so that RO modules operating in a double pass mode, produced 10 gpm of permeate from 40 gpm (gals/min) raw water pumped through a multi-media filter. After filtration through the multi-media filters, the water passes through a housing consisting of forty 5 .mu.m cartridge filters. The solids-free feedwater was fed to a bank of four pressure vessels in series, each vessel containing two 6" (inch) diameter RO membrane elements for a total of 8 elements. Production of permeate had to be interrupted to replace the multimedia filter and to replace the cartridge filters.
The package was mounted on a 5 ton trailer along with a 30 Kw (kilowatt) generator, and 3 open top collapsible fabric tanks able to hold 3000 gals. Two of the tanks are used to store potable water made, and one is used to collect water for back-washing of the filter media and cleaning of the RO elements with adequately high-pressure pumps. Two raw water pumps supply 30 gal/min to the ROWPU and distribution and back-wash pumps are used when required. Upon unloading the equipment at a chosen site, the components of the ROWPU had to be assembled before being placed in operation.
The foregoing unit was transportable with difficulty and was therefore limited for use where a very large wheeled vehicle can be driven, or a very large skid can be hauled. Most particularly, and critical to the invention herein, the many components of the armed forces system, and other prior art units, failed to recognize the importance of distributing the components within a housing in such a manner that the masses of the components were substantially symmetrically disposed relative to the center of mass of the container.
In common with the prior art ADROWPU, the ROWPU screen filters had to be cleaned manually after stopping operation of the unit to make RO permeate. In each case, a source of clean water was required to back-flush the screen filters. In each case, it was a serious drawback with respect to efficient production of RO permeate because permeate was saved to clean the filters.
The problem of back-flushing the screen filters and chemically cleaning the RO membranes "in place" within the RO vessels is particularly acute when water contaminated by NBC contaminants must be purified, since a source of clean water (such as "tap-water" in a city) is not available, and one can ill afford to use permeate for such a function. Moreover, the use of a back-flushing pump and yet another pump for chemically cleaning the RO membranes added to the size and weight of the equipment in the ROWPU.
The novel ADROWPU of this invention (still referred to as such because the new and improved unit is air-deliverable) avoids using a back-flushing pump and a separate chemical cleaning pump. Stored permeate is not used to back-flush the screen filters, as was done in the prior art. Further, the screen filters are back-flushed with concentrate from the RO modules which would have been, and is, discarded.
Most importantly, the screen filters are back-flushed automatically, without stopping production of permeate. This automatic back-flush is accomplished by a unique utilization of a commercially available self-cleaning Filtomat screen equipped with an automatic screen-cleaning means actuated upon being back-flushed. To work efficiently this device is designed to operate with an available supply of at least 110 gals/min at a pressure in excess of 30 psig (&gt;30 psi) for from 3 to 5 secs.
Conventionally, several Filtomat screens are used in parallel in a system where the flow of available water is essentially unlimited. When one of the screens becomes fouled, it automatically requires a diversion of flow from the main feed line. This flow is readily supplied because there is no shortage of volume (in excess of 110 gpm is usually available) at above the required pressure.
In the rare instance where a single Filtomat unit is used, it is not critical to have continuous flow delivered to the system. Under such circumstances, if the feed pressure and flow to the Filtomat is not adequate, the outlet valve on the effluent of the device is automatically closed and not opened until the cleaning cycle is completed.
In an ADROWPU, since the effluent from the Filtomat is the feed to the process pump, it is critical to maintain the flow of 40 gpm and adequate pressure (15 psig) in the line.
If, as in a conventional situation, one was to provide 110 gpm for the Filtomat and still maintain a flow of 40 gpm to the ADROWPU unit, one would require a 150 gpm pump. It is mechanically impractical and unsound to constrain a 150 gpm pump solely to use it at its design throughput once every hour (say) for a duration of from 3 to 5 secs. It would be used at the design throughput even less frequently if the raw water to the unit is not too dirty.
Since the maximum flow in a transportable ADROWPU is 40 gpm, delivered by the feed pump, the conditions of water availability at a site where a ADROWPU unit would operate are far removed from those at a site where such a Filtomat self-cleaning device is conventionally used.
As will be described in greater detail hereafter, the Filtomat screen can only be used in the ADROWPU in combination with an adequately high pressure storage means capable of delivering at least 110 gpm for a short period of time. It is unexpectedly practical to store the concentrate for back-flushing only because, for back-flushing, this concentrate is to be delivered from the storage means (a bladder tank is used), for only from 3 to 5 seconds at a pressure sufficient to ensure cleaning the fouled screen. Thus, except for storing flushing water in a bladder tank under at least 95 psig pressure (so that at the end of the flushing cycle at least 35 psig remains) the Filtomat could not have been used in an ADROWPU.
Another improvement in the new ADROWPU is that no permeate is stored, to be made up into a chemical cleaning solution for fouled RO modules. A "clean-in-place" tank is carried in the ADROWPU, but this tank is empty (saving weight) except when the RO modules are being cleaned with permeate to which cleaning chemicals are added.
Most particularly, the PC of this invention is designed not to use outside sources of water for cleaning of any type, and, as stated hereabove, to position all the components of the PC in a housing so as to secure the relatively heavier components symmetrically about the center of mass of the PC. The method of operating the equipment in the PC, and the effectiveness of each of the combination of system elements in the package, address the inherent deficiencies of the prior art equipment.
Other deficiencies, such as those discovered during operation in adverse weather, whether frigid or hot, have also been overcome. For example, in sub-zero weather conditions, at ambient temperatures below -15.degree. C., cold enough to freeze water in any of the individual items of equipment in the PC, the prior units used by the army did not function because the equipment was inadequately protected by insulation alone. In the novel PC, heated air, ducted over the hot fins of an air-cooled diesel generator (for the electrical power required to run the PC) provides an unexpectedly effective way of maintaining a desirable temperature within the PC. The heat from the diesel may be supplemented with a conventional heat source such as a 10 kW heater, which will come on automatically when the ambient temperature in the PC is below 5.degree. C.